Thermal spraying, i.e. the generic name for a class of processes that allow the depositing molten or semi-molten materials onto a substrate to form a wear or corrosion resistant coating, has been known in various forms for many years. Thermal spraying processes include plasma, flame, arc-plasma, arc and combustion spraying. Arc spraying is a form of thermal spraying which involves feeding two electrically conductive wires towards one another so that an arc is struck between the tips of the wires to melt the wire tips. The molten material is then atomized and sprayed onto a substrate by compressed gas.
This form of thermal spraying is widely used to provide corrosion-resistant coatings on various metallic articles. For example, U.S. Pat. No. 6,156,443 (Dallaire, et al.) discloses coatings that are formed by arc spraying cored wires onto metallic articles exposed to erodent particles. The cored wires are composed of a sheath of ductile metal, such as iron, low carbon steel, or ductile stainless steel, and a core comprising coarse ferroboron particles. The resulting coatings are designed to include iron boride phases having mean sizes equal to or larger than the sizes of the erodent particles.
Another thermal spraying process is described in U.S. Pat. No. 5,837,326 (Dallaire, et al.), which discloses a process for producing composite coatings comprising fine titanium diboride particles or crystals distributed throughout a stainless steel matrix by arc spraying cored wires onto a metallic substrate. The cored wires typically comprise a stainless steel outer sheath and an inner core of compacted powders including titanium diboride and a metal or metal alloy. The particles or crystals in these coatings impart hardness to the soft stainless steel matrix and enhances the resistance of the coatings to hard abrasive media.
The metal components of the cored wires used to form the coatings disclosed in both U.S. Pat. Nos. 6,156,443 (Dallaire, et al.) and 5,837,326 (Dallaire, et al.) are not highly alloyed, and consequently the coatings tend to be susceptible to corrosion attack in certain high temperature corrosive conditions, such as in boiler applications.
Generally, the binder metal (i.e., the metal of the outer sheath) in a wear-resistant coating is critical to the performance of the coating in corrosive conditions such as those encountered in boilers. For example, coatings with iron-based binder alloys, such as Armacor M™, exhibit extensive binder-material corrosion in boiler conditions, resulting in accelerated wear of the coatings. The consequent weakening of the bonds can also lead to premature coating failure due to complete spalling of the protective layer. Furthermore, the magnetic characteristics of these coatings prevent thickness measurements using standard equipment, such as Elcometers.
U.S. Pat. No. 4,741,974 (Longo, et al.) discloses a composite wire for forming wear resistant coatings wherein the wire is formed of an alloy sheath having iron, nickel, or cobalt as a major component. The core of the composite wire is formed of powder that includes boron or boron carbide. Due to its extreme hardness, boron carbide is employed in coatings where wear or abrasion is of primary concern. However, as with other conventional composite wires, in high-temperature corrosive environments, the wear resistant coatings may experience accelerated wear.